In the US, Sprint, Verizon and US Cellular use CDMA. AT&T and T-Mobile use GSM. Most of the rest of the world uses GSM. The global spread of GSM came about because in 1987, Europe mandated the technology by law, and because GSM comes from an industry consortium.
Accordingly, can you put any SIM card into an unlocked phone?
With an unlocked device, a GSM smartphone can be reprovisioned and used on another network simply by popping out the old SIM card and putting in a new one from the new carrier. The carrier doesn't necessarily need to be notified, and you don't need anyone in the store to reprovision your phone.
Do unlocked phones work on Verizon?
Getting an unlocked phone that wasn't specifically made to work with Verizon on its network is tricky. The CDMA/voice issue pretty much ensures you need a device that's made for Verizon. There are a few exceptions. The good news is that all of Verizon's 4G LTE phones come unlocked out of the box.
Can you use a unlocked cell phone on any network?
US Mobile is a GSM carrier. In general, an unlocked GSM phone from AT&T and T-Mobile will work on each other's networks. Older phones from Verizon and Sprint, on the CDMA network, don't operate with a SIM card. That basically means that those phones are restricted and can't be used on another CDMA networks.